The present invention relates generally to the field of radar, and more particularly, to radar pulse expander-compressors utilizing frequency-modulation derived phase coded pulses.
Accurate parameter estimation in radar systems involves the precise determination of a targets' true range R and velocity V with respect to the radar. Accurate range determination requires small range-time resolution cells .DELTA.t. Since the size .DELTA.t of the time resolution cells is determined by the equation .DELTA.t=1/B, where B is the radar pulse bandwidth, an accurate range determination requires a wide bandwidth B. Accurate velocity estimation, in turn, requires two or more samples of target range at different times with the time differences between samples great enough to permit meaningful values of (R.sub.1 -R.sub.2)/(t.sub.1 -t.sub.2).
Range-doppler-coupling waveforms and matched filters (pulse compressors) can be employed to provide radar displays from which accurate target range and velocity can be estimated. A radar can transmit an up chirp on one pulse and a down chirp on the next pulse and compress echoes from each transmission with an appropriate matched filter. Echoes from non-moving targets will compress at the same time after transmission on the two matched filter outputs. However, echoes from targets moving radially toward or away from the radar will compress at different times after transmission in the two compressors due to range-doppler coupling. Range-doppler coupling comprises the addition of a doppler frequency to the echos of moving targets equal to f.sub.D =2 V/.lambda., where V is the radial velocity of the target with respect to the radar, and .lambda. is the radar's carrier wavelength. The time difference between the up and down chirp compressions is directly proportional to the target velocity magnitude and the time duration of the transmitted coded pulse. The midpoint between the two compressed pulses indicates the true target range. The direction of the target velocity can be determined by determining which matched filter produced the shortest time delay between transmission and compression.
As is well known, the product of the transmitted uncompressed waveform T and the bandwidth of the waveform B, determines the pulse compression ratio p. In order to achieve good velocity sensitivity, large pulse compression ratios are required. However, the bandwidth B, which is related to the size of the desired range-time cells .DELTA.t.sub.c, is generally fixed by system parameters. Thus, in order to increase a system's velocity sensitivity, the length of the uncompressed pulse must be increased. Since the time width of the individual code elements (or time cells) in the uncompressed pulse is generally fixed by the bandwidth, an increased uncompressed pulse length must be obtained by increasing the number of code elements in the pulse. However, although a code element increase increases the pulse compression ratio, the Fast Fourier Transform (FFT) hardware required to implement this code element increase is significant. For example, assuming a standard 1 microsecond compressed pulse length (i.e. a 1 microsecond code element size), to obtain a 1 millisecond uncompressed pulse length 1000 code elements are required. In order to implement 1000 code elements, the square root of 1000 FFT points are required. The hardware cost and complexity required to realize such an increase in velocity sensitivity is thus a significant drawback.